Various output methods such as wire-dot recording method, thermal recording method, melt thermal transfer recording method, sublimating dye transfer recording method, electrophotographic method and ink-jet recording method have been developed for the output of computers or the like. Among them, the ink-jet recording method is recognized as a method suitable for a personal use because a plain paper is used as the recording sheet, the running cost is low and the hard ware is compact and inexpensive. Further, since full-color technique and high resolution have been attained recently, the ink-jet recording method has lately attracted attraction as an easy output means and is now being widely spread.
Overhead projectors (hereinafter referred to as "OHP") are widely used for conferences and lectures. Recently, it is demanded to use a color OHP, and investigations have been made on ink-jet recording sheets for OHP.
The recording sheets for OHP are transparent films made of a polyethylene terephthalate or the like and each having a transparent ink-receiving layer. Since an ink prepared by dissolving a dye, an organic solvent, an additive, etc. in water is used for the ink-jet recording method, most recording sheets for OHP each comprises a transparent film having an ink-receiving layer made of a water-soluble polymer such as a polyvinyl alcohol, polyvinylpyrrolidone, water-soluble cellulose derivative or gelatin. Those recording sheets are available on the market.
The properties required of the ink-receiving layer are as follows: minute letters or images can be reproduced; the images have high color densities and the faithful color tone thereof is reproduced; the rapid drying is possible after the printing; the printed image does not fade during the storage; and even when a pile of the printed matters is stored, no blocking is caused.
It is also demanded that the recording sheets are free from adhesion under a high humid condition and that they have a high water resistance so that even when the sheets are brought into contact with water, the printed surfaces are not dissolved therein or the printed letters and images are not blurred. However, it is a very difficult problem to impart the water resistance to the ink-receiving layer. The above-described ink-receiving layer made of the water-soluble polymer has no water resistance at all because this substance is essentially soluble in water. It is an ordinary technique to cross-link the water-soluble polymer with a cross-linking agent to improve the water resistance. However, the inventors have found after investigations that when, the water resistance is to be imparted to the ink-receiving layer by insolubilizing the water-soluble polymer by cross-linking it, the obtained layer becomes to adsorb the ink not so much and the quality of the obtained image is seriously impaired. Even though the cross-linking must be weakened to keep a high quality of the image, the water resistance becomes insufficient in such a case. This phenomenon occurs irrespective of the kinds of the cross-linking agents.
Several methods of imparting water resistance to the ink-receiving layer without the cross-linking were proposed. For example, a method wherein a hydrophilic polymer is mixed with a lipophilic polymer is proposed in Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI" No. Sho 57-102391. However, this method has a problem in that since the hydrophilic high-molecular substance is not highly compatible with the lipophilic high-molecular substance, the transparency of the ink-receiving layer is reduced. Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J. P. KOKOKU") No. Hei 5-23597 discloses a polyvinyl acetal resin for the ink-jet recording sheets. This method is free from the problem of the compatibility unlike that of J. P. KOKAI No. Sho 57-102391 because a high-molecular chain has both hydrophilic functional group and lipophilic functional group, and is characterized in that a transparent ink-receiving layer having a high water resistance can be obtained. However, this resin for the ink-jet method has a defect in that since it is not soluble in water, it must be dissolved in a mixture of water and an organic solvent before the application to the sheet. Another problem of this resin is that a relatively long time is necessitated for drying the ink.
Further, a method wherein active energy rays such as electron beam and ultraviolet rays are used was also proposed. For example, J. P. KOKAI No. Hei 1-229685 discloses an ink-jet recording sheet having a receiving layer having a high water resistance, which is prepared by cross-linking a composition comprising a water-soluble resin, polyethylene glycol di(meth)acrylate and water as the indispensable components with active energy rays. J. P. KOKAI No. Sho 62-94380 discloses an ink-jet recording sheet having an ink-receiving layer prepared by curing a resin composition containing a cationic synthetic resin having a photo-polymerizable double bond, which is cured by the active energy ray. J. P. KOKAI No. Hei 1-286886 discloses an ink-jet recording sheet having an ink-receiving layer prepared by curing an ionizing radiation-curable hydrophilic monomer and/or hydrophilic oligomer in the form of a solution thereof in a solvent by the irradiation with the ionizing radiation. It is common in these prior techniques that the composition containing a double bond which is radical-polymerizable with the active energy ray is applied to a base sheet and then the polymerization and cross-linking are conducted by the irradiation of the active energy ray to form the ink-receiving layer. Although the receiving layer having a high water resistance can be surely obtained by these methods, the image quality obtained by printing with an ink-jet printer is low disadvantageously. Reasons therefor are supposed to be that the cross-linking density is easily increased and that the intended polymer of a high molecular weight cannot be easily obtained because the polymerization time is short.
It is also known to form a receiving layer of a high water resistance by cross-linking a double bond-free resin composition by the irradiation with the active energy ray. For example, J. P. KOKAI No. Hei 7-81211 contains an Example wherein a solution containing only a water-soluble polymer is applied to a base sheet and, after drying, it is irradiated with an electron beam to form the receiving layer. However, after investigations, the inventors have found that the kind of the water-soluble resins which can be cross-linked and water-insolubilized by the irradiation of the electron beam of as small as about 10 Mrad is limited to a very small range and that to sufficiently conduct the cross-linkage, it is necessary to also use a radical-reactive hydrophilic monomer or to also introduce a radical-reactive unsaturated group into the water-soluble polymer when the sufficient cross-linkage is to be conducted.
J. P. KOKAI No. Hei 8-267905 discloses a receiving layer of double layer structure prepared by forming an electron beam-cured inner layer comprising a polyvinyl pyrrolidone and an aqueous electron beam-curable compound as main components on a base sheet, applying an aqueous solution of a water-soluble polyalkylene oxide thereto, irradiating the formed solution layer with electron beam before it is dried, and drying the solution layer to form an outer layer. Although the ink-jet recording sheets having a high water resistance and being capable of providing an excellent image quality can be obtained by this method, it has a defect of a low productivity because the coating and the irradiation with electron beam must be conducted twice. In addition, the polyalkylene oxides have a problem of causing "blurring" of the image.